(1) Field of the Invention
The invention generally relates to a television system for transmitting a picture signal is a digital format via a transmission medium from an encoding station to a decoding station.
More particularly the invention relates to a television system of the type in which picture transform coding is used.
Such a television system may form part of a television broadcasting system, in which case the encoding station forms part of the television broadcasting transmitter and each TV receiver is provided with a decoding station. The transmission medium is the atmosphere in this case.
Such a television system may also form part of a video recorder, in which case the transmission medium is the video tape.
(2) Description of the Prior Art
As is generally known, a picture can be considered to be a two-dimensional array of M.times.N picture elements. If these picture elements are encoded independently of one another, such an array is referred to as the canonical picture. In a 625-line TV picture the visible part of the picture comprises 576 lines and each line comprises 720 picture elements. If the brightness of each picture element is represented, for example, by 8 bits, approximately 3.10.sup.6 bits are required for the representation of the brightness of this canonical picture, which involves a bit rate of approximately 75.10.sup.6 bits/sec for 25 pictures per second. In practice, this is found to be inadmissibly high. By subjecting the canonical picture to a two-dimensional transform, the number of bits per picture and hence the bit rate can be reduced considerably.
For performing such a transform the picture is divided into sub-pictures of N.times.N picture elements each. Each sub-picture is subsequently converted by means of a two-dimensional transform into an array of N.times.N coefficients. The transform has for its purpose to obtain an array of coefficients which are mutually uncorrelated. In how far this object is achieved is closely related to the transformation chosen. In this connection the Karhunen-Loeve transform (see, for example Reference 1, pp. 259-264) is found to be optimum. However, this transform method cannot easily be implemented. Nowadays the discrete cosine transform (DCT) is generally considered to be the best alternative (see Reference 2).
The physical significance of this two-dimensional transform is the following. Each sub-picture is considered as a sum of N.sup.2 mutually orthogonal basic picture B.sub.i,k each also consisting of N.times.N picture elements and each with its own weighting factor y.sub.i,k ; i,k=0, 1, 2, . . . N-1. It is these weighting factors which are obtained by the two-dimensional transform. As is common practice, they will hereinafter be referred to as coefficients. Of the said basic pictures B.sub.o,o is representative of the average brightness of the sub-picture. Therefore, y.sub.o,o is referred to as the DC coefficient. In contrast thereto the other coefficients are usually referred to as AC coefficients.
A reduction of the number of bits to be transmitted per picture is now realized by transmitting only those AC coefficients whose absolute values are larger than a given threshold value. This is known as "threshold sampling" (see page 813 of Reference 3). Unfortunately it must be specified which coefficients are transmitted. This is effected, for example by transmitting of each coefficient to be transmitted the address of the location which it has in the coefficient array. For this purpose more extra bits are required and the bit rate reduction is less impressive. It is to be noted that the DC coefficient is always transmitted.
To realize a further reduction of the bit rate reduction, Reference 4 proposes to arrange the AC coefficients of a coefficient array in the order of decreasing magnitude and to transcode each AC coefficient in a transmission word of variable length, which is dependent on the magnitude of the AC coefficient on the one hand and on its ordinal number in the array on the other hand.